1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a discharge circuit in a nonvolatile semiconductor memory device such as a NAND flash memory.
2. Description of the Related Art
FIG. 1 shows a core portion of a NAND flash memory, and FIG. 2 shows a structure of a NAND cell 1 of FIG. 1. Further, FIG. 3 shows signal waveforms at the time of programming in the NAND flash memory of FIG. 1. Hereinafter, a programming operation of the NAND flash memory will be briefly described with reference to the drawings.
Each of NAND strings 1 comprises, as shown in FIG. 2, a drain-side select transistor 20 connected to a bit line, a plurality of memory cell transistors 21, and a source-side select transistor 22 connected to a cell source line CELLSRC. A gate of the select transistor 20 is connected to a select gate SGD, and a gate of the select transistor 22 is connected to a select gate SGS, respectively. When data is programmed in one of the plurality of memory cell transistors 20 in the NAND string 1, the data to be programmed is transmitted to a selected bit line 5 via a sense amplifier 3. The adjacent non-select bit line 4 is charged to a power supply potential Vdd via a bit line shield line BLCRL. The bit line 4 corresponds to an even-numbered page in a memory cell array, and the bit line 5 corresponds to an odd-numbered page in the memory cell array, respectively. The cell source line CELLSRC is precharged to a potential not ground potential Vss (a Vdd potential or a potential which is lower than Vdd by a threshold voltage of the transistor) in order to suppress a current leakage to the select gate SGS side in channel boosting. When a word line WL is driven and data programming into the memory cell is terminated, a recovery operation is carried out. In the recovery operation, the bit line shield line BLCRL and the cell source line CELLSRC are discharged via discharge circuits 11 and 10, respectively. The discharge circuit 10 is controlled by a control signal CELLSRCVSS, and the discharge circuit 11 is controlled by a control signal BLCRLVSS, respectively (Jpn. Pat. Appln. KOKAI Publication No. 8-87895).
As shown in the signal waveforms of FIG. 3, the cell source line CELLSRC is discharged to Vss via the discharge circuit 10. Substantially at the same timing, the bit lines 4 and 5 are equalized and then are discharged to Vss via the bit line shield line BLCRL and the discharge circuit 11. FIG. 4 shows a row decoder 40 including a SGD driver which drives the select gate SGD, a WL driver which drives word lines WL0 to WL31, and a SGS driver which drives the select gate SGD. The inventors of the present application have found that when the cell source line CELLSRC and the bit lines 4, 5 are discharged, a PN junction in the row decoder 40 is biased in a forward direction to cause a bipolar operation. This is assumed to be based on the following reasons.
FIG. 5 shows a cross sectional view of the select gate SGS and the memory cell transistors in the NAND string. The select gate SGS has strong capacitive coupling of about 20% to 40% to the cell source line CELLSRC formed of a metal wiring M0. Further, the select gate SGD (not shown) has strong capacitive coupling of about 20% to 40% to a bit line formed of a metal wiring (not shown). Thus, when the cell source line CELLSRC and the bit lines 4 and 5 are rapidly discharged to Vss in the recovery operation, the potentials of the select gates SGS and SGD tend to lower from Vss, which is supplied from the driver side, to a negative potential. How much the select gates SGS and SGD lower depends on a discharge rate at which the cell source line CELLSRC and the bit lines are discharged to Vss, the strength of the capacitive coupling of the select gates SGS and SGD, a potential supply capability of the driver, and the like.